The rotational viscometer or rheometer which has a digital readout capability such as among those of the well known LV series (Models LVDV-I+, LVDV-II+ and LVDV-III+) and RV series (Models RVDV-I+, RVDV-II+ and RVDV-III+) available from Brookfield Engineering Laboratories, Inc. (Stoughton, Mass.) and the well known Tannas Basic Rotary (TBR) (Trademarks of the Tannas Co., Midland, Mich.) viscometer available from the Tannas Co. (Midland, Mich.) in general measures torque or drag experienced by a rotating rotor because of the viscosity of a test liquid at low shear through deflection of a sensitive spring. This deflection is converted into appropriate electronic signals which may be displayed as a torque value and/or converted into and displayed as viscosity and/or other relevant values.
As with most instruments, these require calibration.
The longstanding practice in the art for calibration of such an instrument involves the use of calibration fluid(s) of known viscosity. In such calibration, a plot of known viscosity versus measured torque over the range of expected viscosity of the test sample at a specified rotor speed is often employed to convert the measured torque into the true viscosity. Also, for varying test samples, the longstanding practice associated with this in the field of rotational viscometry is to use one of a number of rotating spindles of various sizes depending upon the expected viscosity range of the test sample. In instruments such as these with digital readout capability, the spindle size is displayed and taken into account in calculating for the viscosity or related property displayed.
Accordingly, the following equation has been applied to determine viscosity in such instrumentation and methodology: EQU V=KST/rpm (B)
wherein:
"V" is the viscosity in units of poise;
"K" is a torque constant related to the spring employed;
"S" is a spindle multiplier constant related to the particularly sized spindle employed;
"T" is the torque measured, and
"rpm" is the rotations per minute of the spindle.
Unfortunately, the foregoing method and instrumentation may lack the highest of accuracy. However, increasingly higher accuracy is demanded of viscosity testing, most especially in the low viscosity ranges. Moreover, the cost of instrumentation and the time required for operators to calibrate the instrument and to test fluids with it are of notable concern.
What is lacking and needed in the art is an instrument and method to ameliorate or overcome such problems, thus providing for increased accuracy at reasonable cost, and providing for simplified operation as well.